Add more fractals without subdivide again
No, you cannot change the parameters of an operation after you have modified the mesh with other operations.

What you can do is reopen the dialog of only the previous operator with F6 if you've accdently closed it, but this is not quite what you are looking for.

(An operator is basically every command where you move or change something about a mesh, an object, a material, you name it)

What you want, if I understand you correctly, is to open the operator history with F3 and change the past and have everything that happened after the fractal subdivide to be reapplied on top of your change of the past. This is basically the same as if every operator was a modifier. Modifiers work that way, but operators don't. Once an operator is applied you can't change their old parameters (well, Blender keeps the parameters editable for the last operator).

Maya does almost what you want: It keeps the full operator history of a mesh and you can actually change it afterwards and all the subsequent operators are reapplied. BUT it doesn't work in complicated cases and can lead to crashes :)
Also it's common practice in Maya to regularly collapse (=delete) the operator history, otherwise Maya gets pretty slow.